Interface states in CoFe$_{2}$O$_{4}$ spin-filter tunnel junctions

Spin-filter tunneling is a promising way to generate highly spin-polarized current, a key component for spintronics applications. In this work we explore the tunneling conductance across the spin-filter material CoFe$_{2}$O$_{4}$ interfaced with Au electrodes, a geometry which provides nearly perfect lattice matching at the CoFe$_{2}$O$_{4}$/Au(001) interface.\footnote{P. Lukashev, et al., Phys. Rev. B, \textbf{88}, 134430 (2013).} Using density functional theory calculations we demonstrate that interface states play a decisive role in controlling the transport spin polarization in this tunnel junction. For a realistic CoFe$_{2}$O$_{4}$ barrier thickness, we predict a tunneling spin polarization of about $-$60{\%}. We show that this value is lower than what is expected based solely on considerations of the spin-polarized band structure of CoFe$_{2}$O$_{4}$, and therefore that these interface states can play a detrimental role. We argue that this is a rather general feature of ferrimagnetic ferrites and could make an important impact on spin-filter tunneling applications.